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Old 05-12-2014, 09:46 AM   #1
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Question Wideband AFR and Vacuum Readings on a Stock 1.6 ECU

Let me start by saying that I've completed building a DIYPNP but I need to get a better laptop for tuning before I install it in my car. I plan to swap out the 1.6 ECU and AFM for a speed density setup as soon as that's taken care of.

In the mean time, I'm trying to make sure my 25 year old 125k engine is running properly so that I don't have to troubleshoot more than one problem at a time as learn to work with MS. I've installed an MTX-L wideband O2 sensor with the gage in an eyeball vent and the simulated narrowband wired into the old O2 sensor wire. In the other eyeball vent I've installed a boost/vacuum gauge plumbed to the cruise control port near the throttle body.

Driving around with my spiffy new gauges I've noticed a few things. Is this normal behavior for stock engine management or do I have another problem to solve?

1. I get about 19 in-hg vacuum at idle on cold start, moving to 21-ish once the motor is warmed up. Is this from the rings and valve seals tightening up once warm?

2. When engine braking I see up to ~25 in-hg vaccuum. Is this normal?

3. Revving the engine in neutral when warm, my AFR briefly pegs itself on the lean end of the gauge on tip-in, and on the rich end when I lift. I see much less lag when doing the same thing while in gear. Is this just the inertia of the flapper door and the ECU being slow to respond, or do I have a possible vaccum leak?

Many thanks. I really want to understand this for myself as thoroughly as I can.

Edit: aaaand I made a typo in the thread title. Wonderful.
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Old 05-12-2014, 10:31 AM   #2
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sounds like every normal car...
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Old 05-12-2014, 11:33 AM   #3
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1. Yes
2. Yes, the throttle plate is closed, but the pistons are still moving from the drive of the wheels. This vacuum is how engine braking works.
3. Yes, this is normal. So you're seeing a big difference in AFRs FREE REVVING it in gear vs. neutral? I wouldn't expect so. But the lean tip in followed by the richness once you let off is the delay we live with thanks to the AFM. That's why most of us use TPS based acceleration enrichments, since it adds a little fuel as soon as it sees the throttle move, not once the MAP starts reacting, which is technically a split second later.
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Old 05-12-2014, 11:38 AM   #4
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Yes, all normal.
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Old 05-12-2014, 01:33 PM   #5
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Thank you all. I figured everything was fine but I wanted to confirm my reasoning.

Quote:
Originally Posted by curly View Post
1. Yes
2. Yes, the throttle plate is closed, but the pistons are still moving from the drive of the wheels. This vacuum is how engine braking works.
I got that, I just wasn't sure how close to a perfect vacuum is reasonable. 25 in-hg puts me at roughly 15 KPA. I made a little conversion chart on a post-it note so I could see roughly where I'd be on fuel map under given driving conditions, but it blew away.
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